Nuclear resonance fluorescence (NRF) experiments using linearly polarized γ-ray beams allow for the measurement of level energies and widths, spins, parities, multipolarity mixing ratios, and branching ratios, and, therefore, represent an important tool for probing nuclear structure. These properties are equally important for nuclear astrophysics, since they enter either directly in the determination of thermonuclear reaction rates, or indirectly via energy and efficiency calibrations.
In a study recently published in Physical Review C, we investigated excited states of 40Ca and 11B at level energies between 8 and 9 MeV, using the High-Intensity γ-ray Source (HIγS) at the Triangle Universities Nuclear Laboratory (TUNL). Levels in the 11B nucleus are interesting because they often provide energy and efficiency calibrations for NRF experiments, while excited states in the 40Ca nucleus are important for estimating reaction rate contributions from unobserved low-energy resonances in the 39K(p,γ)40Ca reaction – a reaction which is relevant for improving our understanding of potassium nucleosynthesis mysteries in globular clusters within our own Milky Way. One of the powerful tools available to us at HIγS is the ability to produce photon beams that are linearly polarized and nearly-monoenergetic. These beams give rise to pronounced angular correlations (can be thought of as the “radiation pattern,” or the angles at which γ-rays are emitted from the excited target) that are sensitive to both the spin and parity. As a result, measuring the angular correlation between the incident and emitted γ-rays allows for a straightforward determination of spin and parity values. One of the goals of the present work was to derive precise excitation energies of 40Ca levels that may contribute to the 39K(p,γ)40Ca thermonuclear reaction rate.
In previous literature, the spin-parity of the 8425-keV level in 40Ca is assumed to be 2−. This level then dominates the thermonuclear reaction rates in the 39K(p,γ)40Ca reaction, below a stellar temperature of 100 MK. If this level instead had a spin-parity of 2+ (not entirely ruled out by the previous literature), it would correspond to a dominant rate contribution at even higher temperatures. Additionally, a 2+ assignment would imply a contribution to the 39K(p,α)36Ar reaction rate as well. Since this spin-parity assignment has significant implications for potassium nucleosynthesis in globular clusters – of which there are still particular unsolved mysteries – it must be considered carefully. In our experiment, we obtained improved excitation energies for 40Ca as well as an unambiguous 2− assignment for the state at 8425 keV. We also obtained improved γ-ray multipolarity mixing ratios and more precise branching ratios for the 11B state at 8920 keV.
For more information, see Gribble et al., Phys. Rev. C (2022). https://doi.org/10.1103/PhysRevC.106.014308 . A no-cost preprint version is available at https://doi.org/10.48550/arXiv.2206.13447 . David Gribble is a UNC graduate student.
Figure 1: (Top) The peculiar globular cluster NGC 2419. It is located in the outer halo of the Milky Way, further away than the Small and Large Magellanic Clouds, at a galactocentric distance of 87.5 kpc. The recently observed strong potassium enhancements represent a puzzle for our understanding of both stellar and galactic evolution theory. Credit: ESA/Hubble & NASA. (Bottom) Setup used for our experiment. The incident gamma-ray beam moves inside a plexiglass vacuum tube and impinges on a sample (boron powder or CaO powder). The direction of the linear polarization of the beam points parallel to the horizontal plane. The front face of each detector is covered by a passive shield (yellow) to reduce backgrounds.